This invention relates to image forming apparatus, such as a copying machine, capable of performing image forming operation at a high speed.
With such high speed image forming apparatus, upon a start key being depressed, a photosensitive drum is rotated and simultaneously charged at a specified level by a main charger. A light image reflected from an original document is introduced to a charged surface of the photosensitive drum to form an electrostatic latent image on the surface of the photosensitive drum. To attain high speed image forming operation, the light image is introduced to the charged surface immediately after the charging of the photosensitive drum is started. In other words, the latent image formation is started before the photosensitive drum completes the initial turn.
The latent image is developed into a toner image by electrically attracting toner particles from a developing device. The toner image is transferred by a transfer device from the photosensitive drum surface to a copy sheet which is transported from a cassette into a space between the photosensitive drum surface and the transfer device.
Residual toner is removed by a cleaning device from the surface of the photosensitive drum by a cleaning device after the toner image transfer is completed, and residual charges are removed from the photosensitive drum surface by a removal lamp. Thereafter, the photosensitive drum is again charged at the specified potential level to execute another image forming operation.
However, the residual electric charges on the surface of the photosensitive drum cannot be completely removed even if the luminance and illuminating time of the removal lamp are increased. As shown in FIG. 5, if the photosensitive drum surface which has been charged at V1, e.g., 700 to 800 V, in the initial stage is removed by a removal lamp, there is left a residual potential V2, e.g., 20 to 30 V, on the photosensitive drum surface.
Consequently, such residual potential of the initial charging increases the dark potential of the photosensitive drum surface for the second and following chargings. In other words, there occurs a potential difference between an initial charging portion and a second charging portion (i.e., once charged portion) on the photosensitive drum surface. If an electrostatic latent image and toner image are formed over both the initial charging portion and the second charging portion, the image density of the initial charging portion will become considerably different from that of the second charging portion to display two different portions in tone. This difference is most noticeable in a half-tone image.